As an abundant neurotransmitter in the brain and throughout the body, serotonin plays a significant role in determining many biological processes. Serotonin mediates its actions by binding to and activating G protein-coupled receptors (GPCR) on the surface of neurons and other cells. The activity of a particular GPCR, the serotonin 2A receptor, is associated with several neuropsychiatric disorders and many efforts spanning several decades have been made to modulate the activity of this receptor to improve psychiatric treatments. For example, the atypical antipsychotic drug clozapine was designed to block the activity of this receptor. Additional pharmaceutical therapeutics, such as selective serotonin uptake inhibitors (SSRIs) and related antidepressants, may indirectly affect activity at this receptor by altering brain serotonin levels. The serotonin 2A receptor is also a prominent target for drugs of abuse, particularly those classified as hallucinogens;lysergic acid diethylamide (LSD) mediates its hallucinogenic effects by directly activating this receptor. Other drugs of abuse that indirectly act at the 2A receptor are amphetamines, particularly MDMA or "ecstacy". We are proposing a new basis for drug development targeting the regulation of the serotonin 2A receptor. We have found that serotonin activates the 2A receptor and downstream signaling is mediated by an intracellular regulatory protein called ?arrestin. Regulation of the serotonin 2A receptor in vivo may set the tone for neurological sensitivity to endogenous levels of serotonin as well as the responsiveness to pharmacological agents. Our studies explore the biochemical, physiological and behavioral consequences of disrupting the serotonin 2A receptor-?arrestin interactions in vivo. We predict that drugs that disrupt the serotonin 2A receptor-?arrestin interaction might provide a means to alter the sensitivity of the receptor to the levels of serotonin present in the brain. These findings may inspire the development of drugs that could maintain a desired basal serotonergic tone while eliminating excessive receptor responsiveness to endogenous serotonin. Such strategies in drug design may prove to be clinically useful for treating neuropsychiatric disorders including those associated with drugs of abuse. PUBLIC HEALTH RELEVANCE: Our studies explore the biochemical, physiological and behavioral consequences of disrupting serotonin receptor regulation. We predict that drugs that disrupt this regulation might provide a means to alter the sensitivity of the receptor to the levels of serotonin present in the brain. These findings may inspire the development of drugs that could be clinically useful for treating neuropsychiatric disorders such as schizophrenia as well as those associated with drug addiction.